Currently, a widely available electrophotographic image forming method is a method in which a final image is formed via a fixing process after transferring into a plain paper sheet a toner image on an electrostatic latent image carrier, which is formed via a developing process to visualize an electrostatic latent image with toner, by bringing a charge-provided toner into contact with the electrostatic latent image formed on the electrostatic latent image carrier (usually referred to as an electrophotographic photoreceptor), or by making the toner to face the electrostatic latent image carrier via a narrow spacing.
As development processes to form toner images, there are a double component development process in which toner is charged and developed employing a double component developer composed of a carrier and the toner, and also a single component development process in which a developer consisting of toner is conveyed by a developing roller, and charged via friction with a developer regulating member or such to conduct a development treatment. This single component development process has widely been used in recent years since no carrier needs to be used in this process, and a developing device can also be simplified. With the recent development of colorization, attention has been focused on a non-magnetic single component process employing toner with no content of a magnetic material since the colorization is possible with it.
This process differing from the double component development process has the advantage that the development device mechanism is not complicated, and is easily downsized, since friction of only toner is caused with an electrification member without using a carrier, or electrification is caused by pressing the carrier on the developing roller surface. As the result, it is further a feature that this process is also usable for a color image forming apparatus usually employing at least 4 development mechanisms.
Usually, a developing roller comprising an elastic layer composed of silicone rubber provided around the outer circumferential surface of a conductive shaft, for example, has been utilized for a developing roller with this non-magnetic single component development process. A developing device having a very simple mechanism is to be employed since a toner thin layer is formed on the developing roller using an electrification member such as a metal plate or a roller, and friction is caused with this, in order to charge the toner.
This developing roller comprises an elastic layer composed of a rubber elastic body such as silicone rubber provided around the outer circumferential surface of a shaft (or a spindle) made of a metal or a conductive resin, but it is commonly known that a surface layer is formed on the elastic layer employing other resins, in order to acquire toner conveyance and electrification. It is also known, for example, that a fluorine-containing rubber is employed on an elastic layer in order to provide electrification to toner together with toner conveyance, and to further prevent toner adhesion and fusing. In order to form a fluorine-containing rubber layer on the elastic layer, it is also known that an intermediate layer composed of a silane coupling agent is formed on the elastic layer surface, and an outermost layer composed of a fluorine-containing rubber as a principal component is further formed on the intermediate layer (refer to Patent Document 1).
However, it is desired that a toner layer formed on the developing roller via toner regulating conveyance during image formation is rubbed by an electrification member, or it is strongly pressed against the elastic layer surface to cause electrification. It is common that a plurality of structural layers are provided on the elastic rubber layer in order to protect the outer layer surface from scratches, or to sufficiently produce toner electrification with a desired polarity.
In this case, a strong frictional force to charge a toner layer formed on developing roller is applied, or a pressing force is applied with an electrification member during electrification, whereby a large force is also applied to the developing roller. As the result, the elastic layer on the roller shaft is locally deformed or distorted. A deforming stress applied between this elastic layer and a coated layer provided on it becomes large as a matter of course, whereby peeling or cracking tends to be generated. Accordingly, a technique utilizing a non-magnetic single component development process has been demanded to solve the above-described problems.
The non-magnetic single component development is also capable of receiving and transferring electric charge between the toner and the developing roller, and counter electric charge of the toner is accumulated on the developing roller surface. This counter electric charge is removed by leaking it into the developing roller to constantly neutralize charge on the developing roller surface. However, when the foregoing structure is employed, no charge formed on a surface layer is effectively leaked since an intermediate layer serves as a barrier layer in this case, whereby residual charge on the developing roller surface is increased, resulting in occurrence of a problem such as scattering of toner and so forth.
(Patent Document 1) Japanese Patent O.P.I. Publication 8-190263